JPH05318577A - Biaxially oriented polyester film for magnetic recording medium - Google Patents

Abstract

PURPOSE:To obtain a magnetic recording medium runnable without providing a back coat layer by adding plate particles to a polyester film and setting the distribution curve of projections formed on the surface of the film by the plate particles so as to satisfy a specific formula. CONSTITUTION:In the production of a biaxially oriented polyester film for a magnetic recording medium, 0.05-1.0wt.% of plate particles are added to a polyester film with a surface orientation index of 0.17 or more. the distribution curve of projections formed on the surface of the film by the plate particles is set so as to satisfy formula -16X+4.2<=logY<=-17X+6 [wherein X is a distance (mum) in a height direction from a standard level and Y is the number of projection/mm<2> counted at the time of cutting at the height X] within a range wherein height x (mum) is 0.05mum or more and the number Y of projection/mm<2> is 120/mm<2> or more. The magnetic recording medium runnable without providing a back coat layer is obtained by this biaxially oriented polyester film.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a biaxially oriented polyester film for a magnetic recording medium, and more specifically, it contains plate-like fine particles, and the film surface roughness formed by the particles is within a specific range. The present invention relates to a biaxially oriented polyester film which is useful for producing a magnetic recording medium without a back coat, which is excellent in electromagnetic conversion properties, calendering property, edge damage and running property.

[0002]

2. Description of the Related Art Biaxially oriented polyester films represented by polyethylene terephthalate films are widely used as base films for magnetic recording media because of their excellent physical and chemical properties.

In the biaxially oriented polyester film, its slipperiness and abrasion resistance are major factors affecting the film manufacturing process and product quality. For example, in a high-strength base film for a long-time recording magnetic tape having a surface orientation coefficient of more than 0.17, the stretched area ratio is high, voids are easily formed around the lubricant, and the calendering property is easily deteriorated. If the calendering property is poor, when a magnetic layer is applied to the surface of the biaxially oriented polyester film to produce a magnetic tape, friction between the calendering roll and the film surface is severely generated and shavings are generated, leading to an increase in dropouts.

In order to quantify the calendering property,
It is preferable to reduce the peak stress between the calender roll and the projection formed by the lubricant. The higher the height of the protrusions formed on the surface, the more the protrusions are embedded in the compliant roll of the calender and are more susceptible to the peak stress. Therefore, it is desirable that the projection formed on the film surface has a trapezoidal shape.

Further, the center line average roughness (Ra) must be high in order to improve the slidability, but in order to increase Ra, the addition concentration should be increased or the particle size of the lubricant should be increased. There must be. At this time, increasing the concentration of addition and increasing the particle size increase the number of particles that cause calendering in the former, and increase the sharpening stress received from the calender rolls in the latter, which is not suitable for calendering. is there. Moreover, when the surface becomes rough,
As the electromagnetic conversion characteristics deteriorate and the slipperiness becomes too good, the running of the tape becomes unstable, and edge damage occurs at the flanged roll in the VTR. Therefore, in order to increase the contact area with the friction body and control the friction coefficient, plate-like particles forming trapezoidal protrusions have been added. However, many of these plate-like particles have a broad particle size distribution, and there is a concern that the particles form high protrusions and deteriorate the electromagnetic conversion characteristics of the magnetic recording medium.

[0006]

The present inventor has developed a biaxially oriented polyester film which solves the above-mentioned problems and which has a surface property applicable to the field of magnetic recording capable of running without a back coat. Successful.

Therefore, it is an object of the present invention to provide a high quality polyester base film useful for producing a magnetic recording medium, particularly a magnetic recording medium which can be run without a back coat and has good electromagnetic conversion characteristics, Another object of the present invention is to provide a biaxially oriented polyester film for a magnetic recording medium, which has good calendering property and little edge damage when the tape is running.

[0008]

The object of the present invention is to:
According to the present invention, in a polyester film having a plane orientation index of 0.17 or more, the plate-like particles are added in an amount of 0.05 to 1.0 w.
The distribution curve of the protrusions formed on the film surface by the content of t% is such that the height X (μm) is 0.05 μm or more and the number Y of protrusions (pieces / mm ² ) is 10 pieces / mm ² or more. It is achieved by a biaxially oriented polyester film for a magnetic recording medium, which satisfies the following formula (1) in the range.

[0009]

## EQU3 ## −16X + 4.2 ≦ log Y ≦ −17X + 6 (1)

[Where X is the distance (μm) in the height direction from the reference level, and Y is the number of protrusions (pieces / mm ² ) counted when cut at the height X. ]

The polyester in the present invention is a polyester having an aromatic dicarboxylic acid as a main acid component and an aliphatic glycol as a main glycol component. Such polyesters are substantially linear and have film forming properties, especially film forming properties by melt molding.
Examples of the aromatic dicarboxylic acid include terephthalic acid, naphthalene dicarboxylic acid, isophthalic acid, diphenoxyethane dicarboxylic acid, diphenyl dicarboxylic acid, diphenyl ether dicarboxylic acid, diphenyl sulfone dicarboxylic acid, diphenyl ketone dicarboxylic acid, and anthracene dicarboxylic acid. it can. Examples of the aliphatic glycol include polymethylene glycol having 2 to 10 carbon atoms such as ethylene glycol, trimethylene glycol, tetramethylene glycol, pentamethylene glycol, hexamethylene glycol, decamethylene glycol, and aliphatic diols such as cyclohexanedimethanol. Etc. can be mentioned.

The polyester in the present invention is preferably one having alkylene terephthalate or alkylene naphthalate as a main constituent.

Among such polyesters, especially polyethylene terephthalate and polyethylene-2,6-naphthalate, for example, 80% or more of the total acid component is terephthalic acid or 2,6-naphthalenedicarboxylic acid, and the total glycol component. A copolymer in which 80 mol% or more of ethylene glycol is ethylene glycol is preferable. 20 mol% or less of the total acid component can be the above-mentioned aromatic dicarboxylic acids other than terephthalic acid or 2,6-naphthalenedicarboxylic acid, and alicyclic dicarboxylic acids such as adipic acid and sebacic acid: It can be an aliphatic dicarboxylic acid such as cyclohexane-1,4-dicarboxylic acid and the like. Also, up to 20 mol% of the total glycol component can be the above glycols other than ethylene glycol, or, for example, hydroquinone, resorcin, 2,
Aromatic diol such as 2-bis (4-hydroxyphenyl) propane: Aliphatic diol having an aromatic ring such as 1,4-dihydroxydimethylbenzene: Polyalkylene such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol It can also be glycol (polyoxyalkylene glycol) or the like.

The polyester of the present invention includes
Aromatic oxyacids such as hydroxybenzoic acid, ω-
Also included are those in which a component derived from an oxycarboxylic acid such as an aliphatic oxyacid such as hydroxycaproic acid is copolymerized or bonded at 20 mol% or less based on the total amount of the dicarboxylic acid component and the oxycarboxylic acid component.

Further, in the polyester of the present invention,
An amount in a range that is substantially linear, for example, for all acid components,
It also includes a copolymerized polyfunctional polycarboxylic acid or polyhydroxy compound such as trimellitic acid and pentaerythritol in an amount of 2 mol% or less.

The above polyester is known per se and can be produced by a method known per se.
The polyester has an intrinsic viscosity of about 0.4 when measured at 35 ° C. as a solution in o-chlorophenol.
Of about 0.9 are preferred.

The biaxially oriented polyester film of the present invention has a large number of fine projections on the film surface, as is clear from the explanation of the projection distribution which defines the flatness of the film surface.

According to the present invention, the large number of fine projections originates from the large number of substantially inert solid particles (plate-like particles) dispersedly contained in the polyester.

The distribution curve of the protrusions formed on the film surface by the particles present in the polyester is the height X (μ
m) is 0.05 μm or more and the number of protrusions Y (pieces / mm ² ) is 1
It is important to satisfy the following formula (1) in the range of 0 pieces / mm ² or more.

[0020]

## EQU00004 ## -16X + 4.2.ltoreq.log Y.ltoreq.-17X + 6 (1)

[Where X is the distance (μm) in the height direction from the reference level, and Y is the number of protrusions (pieces / mm ² ) counted when cut at the height X. ]

When the projection distribution curve deviates from the lower limit of the range of the above formula (1), when the film is used as a base film of a magnetic tape without a back coat, the running property becomes insufficient. The presence of the compound makes the surface of the magnetic layer rough, resulting in insufficient electromagnetic conversion characteristics, which is not preferable as a base film for a magnetic recording medium.

When the ratio (SRz / Ra) of the three-dimensional ten-point average roughness (SRz) to the centerline average roughness (Ra) is larger than 20, the projections have a sharp shape and are sharpened by the calender roll. The breaking stress increases, the calendering property deteriorates, and the coefficient of friction in the running system in the VTR is too low, which causes fluctuations in tape running, resulting in damage to the end surface of the magnetic tape, that is, edge damage, which is not preferable.

In the present invention, the projections formed on the surface of the film are mainly derived from plate-like particles. It is preferable that the plate-like particles have a specific shape and size.

The plate-like particles forming trapezoidal protrusions on the film surface have an average particle size of 0.1 to 0.1 as determined by the centrifugal sedimentation method.
It is preferably 1.0 μm. Average particle size is 0.1μ
If it is less than m, the film surface becomes too flat and the coefficient of friction during tape running increases, which is not preferable. This particle size is 1.
If it exceeds 0 μm, the surface of the obtained film becomes too rough, and the electromagnetic conversion characteristics after forming a magnetic tape are deteriorated, which is not preferable.

The plate-like ratio of the plate-like particles (plate diameter of the surface of the plate-like particles / plate thickness) is 5 to 20, more preferably 10 to 10.
Twenty. If the plate ratio of the particles is less than 5, the height-to-width ratio of the trapezoidal protrusions formed on the film surface becomes large, the peaking stress applied to the protrusions at the time of calender becomes large, and the calendar sharpening property deteriorates. In addition, the coefficient of friction during tape running is lowered, and the effect on edge damage is reduced, which is not preferable. On the other hand, if the plate ratio exceeds 20, the slipperiness is undesirably lowered.

An inert powder having a classification-enhanced lubricant plate ratio (plate diameter of the surface of plate particles / plate thickness) of 5 to 20 and an average particle size of 0.1 to 1.0 μm as determined by a centrifugal sedimentation method. The addition amount of the plate-like inorganic particles is 0.1 to 1.0 wt%. If the addition amount of the plate-like particles is less than 0.1 wt%, the center line average roughness (Ra) will be lowered, and the slipperiness will be insufficient. on the other hand,
If it exceeds 1.0 wt%, overlapping of the added particles occurs,
It is not preferable because shavings are easily generated under various wear conditions. A more preferable addition amount is 0.1 to 0.4 wt%. The particle size distribution ratio of particles to be added (particle size ratio of cumulative weight fraction by centrifugal sedimentation method is 80% and 20%) is 2.
If it is larger than 5, the number of particles forming coarse projections increases, the distribution of projections becomes broad, and the electromagnetic conversion characteristics are deteriorated. Therefore, the particle size distribution ratio is preferably 2.5 or less.
Examples of the plate-like particles include kaolin, kaolinite, halosite, metahalloysite, dickite, kaolin minerals containing nacrite, china clay, synthetic kaolin, and plate-like alumina. Among them, classification-reinforced kaolin is preferable.

In the present invention, in addition to plate-like particles forming trapezoidal protrusions on the film surface, other inert inorganic fine particles and /
Alternatively, heat resistant polymer particles may be added.

The types of the inert inorganic fine particles and the heat-resistant polymer particles are not particularly limited, and examples thereof include inorganic inert particles such as calcium carbonate, silicon dioxide, titanium oxide, aluminum oxide and kaolin, and silicon particles. The particles may be organic inert particles such as crosslinked polystyrene particles or particles internally deposited by a polymerization catalyst. Even when the inert inorganic particles and / or the heat resistant polymer particles are used in combination, it is important that the surface of the formed film satisfies the above formula (1).

The biaxially oriented polyester film of the present invention has, for example, a melting point (Tm: ° C.) to (Tm + 70) ° C.
Melts the polyester at the temperature of
An unstretched film of 0.9 dl / g is obtained, and the unstretched film is uniaxially (longitudinal or lateral) (Tg-10)-
Stretching at a temperature of (Tg + 70) ° C. (however, Tg: glass transition temperature of polyester) at a draw ratio of 2.5 to 5.0,
Then, in the direction perpendicular to the stretching direction (if the first stage stretching is the longitudinal direction, the second stage stretching is the transverse direction), Tg (° C) to (T).
It can be produced by stretching at a temperature of g + 70) ° C. at a draw ratio of 2.5 to 5.0 times. In this case, the area draw ratio is 9 to 2
It is preferably doubled, and more preferably 12 to 22 times, but it is necessary that the surface orientation coefficient is 0.17 or more. The stretching means may be simultaneous biaxial stretching or sequential biaxial stretching. Furthermore, the biaxially oriented film has a (Tg + 70) ° C.
It can be heat set at a temperature of Tm (° C.). For example, for polyethylene terephthalate film,
It is preferable to heat-set at 230 ° C. The heat setting time is, for example, 1 to 60 seconds.

The biaxially oriented polyester film of the present invention has good process suitability even under severe conditions of use in the magnetic tape manufacturing process, and has excellent surface smoothness as a magnetic tape and electromagnetic conversion. It has excellent characteristics and good edge damage, and is extremely useful as a magnetic recording medium.

Various physical properties and characteristics in the present invention are measured and defined as follows.

(1) Average particle size and particle size distribution ratio of particles The average particle size and particle size distribution ratio of particles are measured using a centrifugal sedimentation permeation type particle sizer manufactured by Shimadzu Corporation.

(2) Average plate-like ratio of particles The average plate-like ratio of particles means that a film containing the particles to be measured and stretched at least one of lengthwise and widthwise by a factor of 4 or more is sliced with a microtome, and a section is taken with a transmission electron microscope. (Hitachi TEM H-800) was observed at a magnification of 20,000 times, the major axis (plate diameter) and the plate thickness of at least 30 particles visible in the section were measured, and the ratio (plate diameter / plate thickness) ) Refers to the average value.

(3) Refractive Index The sodium D line (589 nm) is used as a light source and the refractive index is measured using an Abbe refractometer. Methylene iodide is used as the mount solution, and measurement is performed at 25 ° C. and 65% RH. The plane orientation index is expressed by the following equation.

[0036]

## EQU5 ## ΔP = (nMD + nTD) / 2-nZ

[Where nMD: refractive index in the vertical direction, nT
D: refractive index in the lateral direction, nZ: refractive index in the thickness direction. ]

(4) Flatness of film surface (4.1) Center line average roughness Ra The center line average roughness Ra is measured according to JIS B 0601. Stylus-type surface roughness meter (SURF manufactured by Tokyo Seimitsu Co., Ltd.)
COM3B), make a chart (film surface roughness curve) under the condition that the needle radius is 2 μm and the load is 0.07 g.
A portion having a measurement length L is extracted from the obtained film surface roughness curve in the direction of its center line, the center line of the extracted portion is taken as the X axis, and the direction of longitudinal magnification is taken as the Y axis.
= F (x), the value (R
a: μm) is defined as the flatness of the film surface.

[0039]

[Equation 6]

In the present invention, the reference length is 0.25 mm and 8
Ra is expressed as an average value of 5 pieces obtained by measuring 3 pieces from the largest value. (4.2) Asperity distribution As for the asperity distribution, 3E-30K manufactured by Kosaka Laboratory Ltd. is used, needle diameter 2 μmR, needle pressure 30 mg, measurement length 1 mm, sampling bitch 2 μm, cutoff 0.25 mm, vertical magnification 2
The projection profile on the film surface is three-dimensionally (three-dimensionally) imaged under the conditions of 10,000 times magnification, 200 times magnification in the lateral direction, and 150 scanning lines.

When the profile is cut in a plane perpendicular to the thickness direction of the film, the plane in which the total cross-sectional area of each protrusion is 70% of the area of the measurement region of the film is a reference level (0 level). And the number of protrusions counted when cut on a plane parallel to the plane of the reference level and separated by a distance X (μm) in the height direction of the protrusions is Y (pieces / mm ² ). X is sequentially increased or decreased, the number of Y at that time is read, and the projection distribution curve is drawn by plotting it on a graph. (4.3) 10-point average roughness SRz For the 10-point average roughness SRz, take 10 points each of the peak and valley of the roughness data of (4.1) above, and set the difference of each sum to 1
Use the value divided by zero.

(5) Calendering Scraping A three-stage mini super calendering device (made by Yuri Roll Co., Ltd.) having an elastic roll made of nylon was used at a heating roll temperature of 80 ° C., a linear pressure of 160 kg / cm, and a speed of 120 m / min. 150
A film slit to a width of mm is treated for 5000 m, and the sharpness is evaluated by the stain condition on the elastic roll. <Judgment> A: No stain is observed on the surface of the elastic roll. ◯: Although the gloss of the surface of the elastic roll is slightly lowered, no shavings are attached. Poor: Adhesion of scraped powder is recognized on the surface of the elastic roll.

(6) Edge Damage To measure the edge damage, the base film was made into a magnetic tape with BR6400 manufactured by JVC, and the tape was run for 50 passes. Then, the end face was observed with a scanning electron microscope to check for scratches.
The evaluation criteria are as follows. ⊚: No scratches are found. ○: There are some rubbing marks, but no actual damage. Δ: Scratches are included, but there is no actual damage. X: Damaged to the extent that it can be visually observed.

(7) Electromagnetic conversion characteristics of magnetic coating film α-FeO ₂ obtained by thermally decomposing needle-shaped α-FeOOH containing 5% of cobalt is hydrogen-reduced to obtain black ferromagnetic metal powder. .. Specific surface area of this ferromagnetic metal powder (BE
(Measured by T method with N ₂ gas adsorption method) is 40 m ² / g.

100 parts by weight of the above ferromagnetic powder and the following composition are kneaded and dispersed in a ball mill for 12 hours.

[0046]

Further, 75 of a triisocyanate compound is used.
% Ethyl acetate solution (7 parts by weight) is added, and the mixture is subjected to high-speed shear dispersion for 1 hour to prepare a magnetic coating solution.

The resulting coating solution is applied on a polyester film having a thickness of 10 μm so that the dry film thickness is 3.0 μm, then subjected to orientation treatment in a direct current magnetic field and dried at 100 ° C. After drying, it is calendered to 1/2
Slit to inch width to get magnetic tape for video.

The electromagnetic conversion characteristics (chroma S / N) of this magnetically coated tape are measured by the following method.

A 50% white level signal (100% white level signal has a peak: two: peak voltage of 0.714 bottles) and a 100% chroma level signal superimposed on it is recorded by using a commercially available VTR for home use. Then, the reproduced signal is measured using a Shibasoku noise meter type 925R.
The definition of chroma S / N is as follows according to the definition of Shibasoku.

[0051]

[Equation 7] Chroma S / N [dB] = 20log ES (P-P) / EN (rms)

Here, ES (PP) is the voltage difference (PP) of the reproduction peak: two: peak of the white level signal.

[0053]

[Equation 8] ES (P-P) = 0.714V (P-P)

EN (rms) is the square root value of the voltage of the reproduced signal of the chroma level signal.

[0055]

[Equation 9] EN (rms) = AM noise effective value voltage (V)

[0056]

EXAMPLES The present invention will be further described below with reference to examples.

[0057]

EXAMPLE 1 Kaolin (average particle size: 0. 0) classified and strengthened in 90 parts by weight of ethylene glycol (hereinafter abbreviated as EG).
(3 μm) was added, and then mixed and stirred to obtain a slurry.

Then, 100 parts by weight of dimethyl terephthalate and 70 parts by weight of EG are mixed with manganese acetate tetrahydrate 0.035.
Kaolin obtained by the above-mentioned method after transesterification in a conventional manner using parts by weight as a catalyst (concentration: 0.2 wt% vs. polymer)
Was added with stirring. Subsequently, 0.03 part by weight of trimethyl phosphate was added, and then polycondensation reaction was carried out under a high temperature vacuum in a usual manner to obtain polyethylene terephthalate pellets having an intrinsic viscosity of 0.62.

This polyethylene terephthalate (hereinafter,
Pellets containing calcium carbonate (average particle size 0.58 μm) and true spherical silica (average particle size 0.42 μm) obtained in the same manner as the pellets are kaolin / calcium carbonate / true spherical Silica = 0.3 /
It is mixed so that it becomes 0.1 / 0.1 (wt%), and 170
After drying for 3 hours at ℃, supply to the extruder hopper and melt at 28
It was melted at 0 to 300 ° C., and the molten polymer was molded and extruded on a rotary cooling drum having a surface finish of about 0.3 S and a surface temperature of 20 ° C. through a 1 mm slit die and rapidly cooled to obtain an unstretched film having a thickness of 144 μm. ..

90% of the unstretched film thus obtained
It was stretched 4.5 times in the longitudinal direction at 90 ° C. and then 3.2 times in the transverse direction at 90 ° C. Then, this biaxially oriented film is
The biaxially oriented film was heat treated for 5 seconds at a temperature of 5 ° C., and the heat treated biaxially oriented film was subjected to a longitudinal relaxation treatment under the conditions of 120 ° C. and a tension of 30 kg / cm ² (film cross-sectional area). The thickness of the obtained biaxially oriented film was 10 μm.

The film thus obtained was coated with a magnetic paint and evaluated. The results are shown in Table 1. The calendering property is good and there is no edge damage due to unstable running. The distribution of protrusions on the film surface is shown in Fig. 1.
As shown by the curve A in Fig. 5, there is no big protrusion and the electromagnetic conversion characteristics are chroma S / N V using the standard base film.
+1.5 dB or more was sufficient as compared with the TR tape.

[0062]

Example 2 The concentration of kaolin (average particle size 0.3 μm), which was classified and strengthened in the base film manufacturing method of Example 1, was 0.1 wt%, and further calcium carbonate (average particle size 0.5) was used.
The concentration of 8 μm) was changed to 0.15 wt%, and the concentration of true spherical silica (average particle size 0.42 μm) was changed to 0.1 wt%. Further, a magnetic tape was produced in the same manner as in Example 1 using the obtained biaxially oriented film. These evaluations were performed and the results are shown in Table 1. The projection distribution curve on the film surface is shown as curve B in FIG.

In this film, the frequency of trapezoidal projections was reduced and the frequency of normal projections was increased, so that the calendering property was inferior to that of Example 1, but was within the allowable range. In addition, since the frequency of high protrusions is higher than that of the first embodiment, the chroma S / N is the first embodiment.
It was worse, but within the acceptable range.

[0064]

Example 3 The concentration of kaolin (average particle size 0.3 μm), which was classified and strengthened in the production of the base film of Example 1, was 0.1 wt%, and the concentration of true spherical silica was 0.2 wt.
% (Calcium carbonate was not added). Further, a magnetic tape was produced in the same manner as in Example 1 using the obtained biaxially oriented film. These evaluations were performed and the results are shown in Table 1. The projection distribution curve on the film surface is shown as curve C in FIG.

The calendering property of this film, the edge damage when the tape was running, and the electromagnetic conversion characteristics were good. However, if the film surface is further flattened, running properties will be hindered and sliding noise due to friction between the magnetic surface and the head surface will be a problem, so care must be taken.

[0066]

Comparative Example 1 Calcium carbonate (average particle size 0.58 μm) and true spherical silica (average particle size 0. 0) were added without adding classification-enhanced kaolin (average particle size 0.3 μm) in the base film manufacturing method of Example 1. The surface was formed by a two-component system of 42 μm). The concentration of calcium carbonate (average particle size 0.58 μm) was 0.2 so that Ra was the same as in Example 1.
0 wt% and spherical silica (average particle size 0.42
(μm) was 0.1 wt%. Using the obtained biaxially oriented film, a magnetic tape was manufactured in the same manner as in Example 1. These evaluations were performed and the results are shown in Table 1. The projection distribution curve on the film surface is shown by curve D in FIG.
Show as.

On the surface of this film, trapezoidal protrusions were not included, and the frequency of protrusions was usually high, so SRz / Ra was high, and the calendering sharpness and edge damage were not very good. In addition, the chrominance S / N is also inferior to that of Examples 1 and 2 because the ridge is drawn on the high protrusion side of the protrusion distribution.

[0068]

[Comparative Example 2] Kaolin having a particle size distribution ratio of 2.7 before classification was added in place of the classification-enhanced kaolin in the production of the base film of Example 1, and the same film formation and evaluation as in Example 1 were carried out. did. The results are shown in Table 1. In addition, a projection distribution curve on the film surface is shown as a curve E.

This film was excellent in calendering property and edge damage, but because the particle size distribution of the particles was broad, high protrusions were formed on the film surface, resulting in a chroma S / N ratio.
Caused the decline of.

[0070]

Comparative Example 3 0.25 wt% of calcium carbonate was added as additive particles for forming surface protrusions in the production of the base film of Example 1, and the film formed was used to perform magnetic recording in the same manner as in Example 1. A tape was manufactured. These evaluations were performed and the results are shown in Table 1. The projection distribution curve on the film surface is shown as curve F in FIG.

The electromagnetic conversion characteristics of this film were within the allowable range, but the calendering property and edge damage were not very good. In particular, since SRz / Ra was large, traveling became unstable, and it was inferior to Examples 1 to 3 in terms of occurrence of edge damage.

[0072]

[Table 1]

[0073]

By using the biaxially oriented polyester film of the present invention, the calendering property is good, the tape running is stable, the edge damage does not occur, the electromagnetic conversion characteristics are good, and the magnetic film can run without a back coat. A recording medium is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the height (μm) of protrusions on the film surface and the number of protrusions.

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Claims (4)

[Claims] 1. A polyester film having a plane orientation index of 0.17 or more containing 0.05 to 1.0 wt.% Of plate-like particles.
%, The distribution curve of the projections formed on the film surface by the particles is such that the height X (μm) is 0.05 μm or more and the number Y of projections (pieces / mm ² ) is 10 pieces / mm ² or more. 2. A biaxially oriented polyester film for a magnetic recording medium, which satisfies the following formula (1). −1X + 4.2 ≦ log Y ≦ −17X + 6 (1) [where X: distance in the height direction from the reference level (μ
m), Y: represents the number of protrusions (pieces / mm ² ) counted when cut at height X. ] 2. The plate-like particles have an average plate-like ratio (plate diameter / plate thickness in the plate-like particles) of 5 to 20, and an average particle diameter corresponding to an area circle of 0.1 to 1.0 μm. The biaxially oriented polyester film according to claim 1, further comprising plate-like particles having a particle size distribution represented by the following formula (2) of 2.5 or less. ## EQU2 ## Particle size distribution ratio = D ₈₀ / D ₂₀ (2) [where D ₈₀ : cumulative weight fraction by centrifugal sedimentation method is 80
Particle size in%, D _20: cumulative weight fraction by centrifugal sedimentation method represents a particle diameter at 20%. ] 3. The biaxially oriented polyester film according to claim 1, wherein the ratio (SRz / Ra) of the center line average roughness (Ra) and the three-dimensional ten-point average roughness (SRz) is 20 or less. 4. The biaxially oriented polyester film according to claim 1, 2 or 3, wherein the plate-like particles are one or more kinds selected from kaolin, chai clay, synthetic kaolin and plate-like alumina.